1. Field of the Invention
This invention relates to an autofocus method and an autofocus system applied to, for instance, a digital camera.
2. Description of the Related Art
There has been known, as an autofocus system used in an imaging device such as a digital still camera or a digital video camera, an autofocus system which adjusts the focus by driving the image taking optical system of the imaging device according to an image signal obtained from a solid image taking element such as a CCD. As the method of adjusting the focus according to an image signal, there have been a contrast autofocus system and a peak autofocus system. In the contrast autofocus system, high-frequency components of the image signal are used, whereas in the peak autofocus system, high-frequency components of the brightness signal of the image signal are used.
In the contrast autofocus system, high-frequency components are extracted from the image signal, and the focusing lens is driven so that the evaluation on contrast obtained by integrating the extracted high-frequency components is maximized. This system is advantageous in that since the focusing point is determined on the basis of the sharpness of the subject, the focus can be accurately adjusted irrespective of the distance to the subject and at the same time, since high-frequency components are integrated within a range finder frame, autofocusing action is stabilized and an optimal focusing position can be readily detected.
However, the contrast autofocus system is disadvantageous in that since the imaging device is controlled so that the averaged contrast in the frame is maximized, it is generally difficult to correctly adjust the focus in the case where the subject is a night view including point light sources or of high brightness though in the case of an ordinary subject, the focus can be correctly adjusted.
Whereas, in the peak autofocus system, high-frequency components are extracted from the brightness signal of the image signal, the focusing position is detected by detecting the maximum value of the high-frequency components of the brightness signal of the image signal. The peak autofocus system is advantageous in that the focus can be adjusted more accurately than the contrast autofocus system when a high-brightness point exists in the frame.
Accordingly, there has been proposed, in Japanese Unexamined Patent Publication No. 2001-141989, an autofocus system in which in order to use the advantages of both the systems, one of the contrast autofocus system and the peak autofocus system is selectively used according, for instance, to the peak value of the brightness signal of the image signal representing the subject, or to the average of the brightness signal.
However, since the peak autofocus system is disadvantageous in that the output evaluation is small and the stability of the autofocusing action is bad, the autofocus system proposed in Japanese Unexamined Patent Publication No. 2001-141989 is disadvantageous in that the stability of the autofocusing action is bad when it operates in the peak autofocus system.